Magnesium granules, particularly salt-coated magnesium granules, are used in the iron and steel industry to effect desulfurization. As one example of this practice, iron blast furnace hot metal may be desulfurized by injecting salt-coated magnesium granules into the melt through a lance using flowing pressurized inert gas as a carrier. To insure that the granules flow freely through the lance, the granules are preferably rounded in shape, such as spheroidal. The terms "spheroid" or "spheroidal" used herein are meant to include such rounded shapes as spheres, ellipsoids of revolution, polygons with rounded corners, etc., that are characterized as being free flowing.
For convenience, the term "magnesium" shall be used hereinafter to cover magnesium per se and alloys based on magnesium.
According to U.S. Pat. No. 4,186,000, it has been known for many years in the electrolytic production of magnesium that the presence of boron as an impurity in the molten salt electrolyte is detrimental to the complete coalescence of magnesium formed during electrolysis. Because of the presence of boron, the magnesium not coalesced into a separate molten phase remains dispersed as droplets in the molten salt and in the cell sludge. When the cell salt or cell sludge is removed from the cell and freezes, the droplets of magnesium solidify into rounded granules trapped in the matrix of salt or sludge.
The problem of molten magnesium not coalescing during electrolysis can be partially alleviated by lowering the amount of boron contained in the feed materials charged to the electrolysis cells. Typically, the cell electrolyte generally contains less than 10 ppm (0.0010%) boron.
Further, the addition of fluorides (as CaF.sub.2, NaF, etc.) to the cell electrolyte has been found to aid in the coalescing of molten magnesium. As Kh. L. Strelets points out in his book Electrolytic Production of Magnesium (translated from Russian to English by J. Schmorak, published in 1977 by Keter Publishing House Jerusalem, Ltd., available from the U.S. Department of Commerce), the mechanism of the fluoride effect is not clear, but fluorides do enhance the coalescence of drops of fused magnesium. Strelets states that most authors recommend the addition of 0.3-0.5% CaF.sub.2 to the electrolyte.
The process described in U.S. Pat. No. 4,186,000 was developed to produce salt-coated magnesium granules for use as inoculants in steel making. Because all molten salt mixtures are not necessarily conducive for providing dispersed magnesium granules, the process comprises adding a boron-containing dispersing agent to the molten mixture of salt and magnesium. The molten mixture of salt and dispersed magnesium is cooled to solidification, suitably ground, and the magnesium granules separated from the fines such as by screening.
Attention is also directed to U.S. Pat. No. 4,279,641 which is a continuation-in-part of U.S. Pat. No. 4,186,000 but differs in that magnesium granules are produced without adding a dispersing agent but by carefully controlling salt compositions.
Materials other than boron can prevent coalescence of magnesium in a molten mixture of salt or sludge and magnesium and leave the magnesium in a dispersed state. Reding and Erickson ("Factors Controlling Melt Loss in Magnesium Die Casting", Paper No. 101, 6th SDCE International Die Casting Congress, Nov. 16-19, 1970) found an ". . . inability of the metal phase in sludge to coalesce because of the presence at the metal flux interface of products of reaction between the alloy and the die casting lubricant."
It has been observed that magnesium oxide can act to prevent coalescence of molten magnesium metal in a chloride salt melt. In Strelets, it is noted that the small amounts of magnesium oxide present in electrolytes tends to become concentrated on magnesium droplets. The number of tiny magnesium droplets increases and magnesium losses increase because the droplets become coated with magnesium oxide and do not coalesce.
Magnesium oxide in the electrolyte salt can arise inadvertently from many sources such as (1) being present in the cell feed materials, (2) reaction of magnesium or magnesium chloride with impurities, (3) burning of magnesium in air, and (4) reaction of air or moisture in the air with the salt electrolyte. Unless special precautions are taken with respect to equipment design and operating practice to avoid all of these sources of magnesium oxide, it may be present to a greater or lesser degree in melts of salt and magnesium metal. Thus, simply agitating a molten mixture of salt and magnesium to form tiny magnesium droplets may result in a stable dispersion of magnesium droplets in the salt matrix where magnesium oxide is inadvertently present. However, the results are not always consistent and the size of the droplets may vary over a broad range. A desirable average size is one ranging from about 10 mesh to about 100 mesh.
It would be desirable to provide a process for directly and efficiently producing spheroidal granules of magnesium over a wide range of conditions.